Physicists build tractor beam using hollow laser

By Edd Gent

Published Tuesday, October 21, 2014

Laser physicists have built a tractor beam that can attract and repel objects using a hollow beam that is bright around the edges and dark in its centre.

The device is the first long-distance optical tractor beam and was able to move particles one fifth of a millimetre in diameter a distance of up to 20cm – roughly 100 times further than previous experiments.

Unlike previous techniques, which used photon momentum to impart motion, the tractor beam created by a team from the Australian National University (ANU) relies on the energy of the laser heating up the particles and the air around them.

The researchers demonstrated the effect on gold-coated hollow glass particles, but they believe the effect could be used in applications such as controlling atmospheric pollution or the retrieval of tiny, delicate or dangerous particles for sampling.

"Demonstration of a large scale laser beam like this is a kind of holy grail for laser physicists," said Professor Wieslaw Krolikowski, from the university’s Research School of Physics and Engineering.

The new technique, described in an article published in Nature Photonics is versatile because unlike previous methods it requires only a single laser beam and the team are confident that the technology could be scaled up to deal with larger objects.

"Because lasers retain their beam quality for such long distances, this could work over metres. Our lab just was not big enough to show it," said co-author Dr Vladlen Shvedov.

The technology works by trapping particles in the dark centre of the beam and as energy from the laser hits the particle and it is absorbed creating hotspots on the surface.

Air particles colliding with the hotspots heat up and shoot away from the surface causing the particle to recoil in the opposite direction. To manipulate the particle, the team move the position of the hotspot by carefully controlling the polarisation of the laser beam.

"We have devised a technique that can create unusual states of polarisation in the doughnut shaped laser beam, such as star-shaped (axial) or ring polarised (azimuthal)," said co-author Dr Cyril Hnatovsky.

"We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will."